1. Fields of the Invention
The present invention relates to a surface plasmon resonance (SPR) measuring device, especially to a SPR measuring device in which the wavelength of an incident beam from a circularly polarized heterodyne light source can be modulated.
2. Descriptions of Related Art
Biosensors are devices for biomolecular interaction analysis, detecting changes of interface properties caused by a specific interaction between specific molecules and target analytes on the sensor interface. Along with development and integration of electro-optical techniques with micro-electromechanical system (MEMS), plurality of biosensing techniques such as confocal laser scanning fluorescence microscopy (CLSFM), Quadtz crystalmicro-balance (QCM), SPR, etc have been developed. SPR has high sensitivity for detection of interface changes and hence has received a great attention from scientists. The SPR properties have been extensively studied and various applications such as biological detections have been developed. SPR instrumentation has been commercialized by a number of companies and SPR techniques have been used for real-time detection of biomolecular reactions.
Since B. Liedberg applied SPR to gas detection and biosensing in 1983, analysis systems based on SPR have been broadly used in various fields. For example, the SPR technique combine with biochips is used in biomedical applications with advantages of label free, immediate detection and high sensitivity. Thus SPR has become a hot topic in research of nano science and biomedical science. SPR is divided into three groups according to the principle of measurement. There are three kinds of SPR detection:
1. angular modulation
2. wavelength modulation
3. phase modulation
The angle of SPR can be determined by using the angular modulation, which is kept constant and the angle of incidence is varied, then the sharp dip appears at a specific angle so as to find out the angle of SPR. As to another method-wavelength modulation, the angle of the incident beam is kept constant and the wavelength is varied. In this method, SPR occurs at a specific wavelength. The SPR parameter (angle or wavelength) depends on the refractive index of the dielectric medium. Change in refractive index changes the value of the SPR parameter. According to changes of the angle and wavelength of SPR, the refractive index or interface bio-reaction is detected. The advantages of the angular or wavelength modulation are a simple structure and an easily-obtained experiment results. But the measurement resolution is not enough. On the other hand, the phase modulation has high measurement resolution. This is due to the phase of the SPR reflected light is highly sensitive to biomolecular interactions at the interface. Although the angular or wavelength modulation is simple and most of SPR equipments are based on these methods, the phase-based detection has become a mainstream because that the concentration of the analyte is much lower and users has higher requirements of the resolution and detection speed.
The SPR phase variation is measured by the heterodyne interferometry, or the phase-shift interferometry (PSI) technique. Refer to a journal article-“Surface plasmon resonance phase shift inter ferometry: Real-time DNA microarray hybridization analysis” (S.-J. Chen*, Y.-D. Su, F.-M. Hsiu, C.-Y. Tsou, and Y.-K. Chen, “Surface plasmon resonance phase shift interferometry: Real-time DNA microarray hybridization analysis,” Journal of Biomedical Optics, vol. 10, no 3,034005, May/June 2005.), reported by a research team of Chen, Shean-Jen, SPR-PSI is a novel technique used to measure the spatial phase variation caused by biomolecular interactions upon a sensing chip. The SPR-PSI imaging system has an enhanced detection limit of 2.5×10−7 refractive index change and a long-time phase stability of π/100 in 30 minutes. Also refer to Taiwanese Pub. App. No. 555972, dated Oct. 1, 2003, “SPR heterodyne interferometry measuring device and method” by Chien Chou, Wen-Chuan Kuo, and the device can detect 0.2 nM concentration changes. Although these techniques can detect tiny concentration and the refractive index, the instruments are high cost. For example, heterodyne light sources or piezoelectric actuators are required. Take PSI as an example, beside the piezoelectric actuator, the sampling time depends on the movement of the piezoelectric actuator. Thus the method can't be used in real-time measurement. Moreover, the phase shift drifts easily due to environmental change. Thus there is a need to provide a novel device that overcomes shortcomings of the above prior arts.
In order to improve above shortcomings of prior arts, the present invention provides a concept of phase differential detection. By a circularly polarized heterodyne light source generated from wavelength modulation in combination with SPR, a phase differential type of SPR measuring device has been developed.